This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Progressive and frequently debilitating myopathy is the major cause of morbidity in patients with type IIIa glycogen storage disease GSDIIIa. While severe disability usually does not occur until the 3rd to 4th decades of life, elevations in serum creatine kinase and aldolase concentrations often occur in young children consistent with progressive muscle damage. The elevation in muscle enzymes worsens during exercise, and restriction of exercise may decrease the progression of muscle damage. The goal of this study is to better understand the cause of exercise-related muscle injury and to test a method to prevent it.